In the filed of signal receivers, converting both broadband and baseband analog signals to digital signals involves an inherent trade-off between Analog to Digital Converter (ADC) sample rate and accuracy. Designers are faced with the choice of using a faster ADC that may lack a high degree of accuracy or using a lower sample rate ADC that has more accuracy. Oftentimes, the choice is made for the designer because the frequency of the received signal, Fsignal, dictates the minimum sample rate, Fsample, that must be used to avoid aliasing. Typically this would be 2Fsignal=Fsample.
One approach that has seen some success in the conversion of Intermediate Frequency (IF) signals is to use alternating ADCs that each sample at half of the desired sample rate (assuming that the bandwidth of the IF signal is less than or equal to double the first Nyquist intervals of each of the ADCs). First, the IF signal is converted into one In-Phase (I) and one Quadrature (Q) signal (i.e., I/Q baseband signals). The I and Q signals are then each digitized by one of a pair of alternating ADCs that sample at one half Fsample. Distortion is added to the signals because of non-ideal and non-matching frequency responses of the two ADCs. In fact, the frequency response mismatch of the two ADCs can eliminate much of the advantage of a two ADC system over a single ADC system.
In one solution, the I and Q signals are then processed by local oscillators that multiply the I digitized signal by a sequence of [1, −1, 1 . . . ] and multiply the Q digitized signal by a sequence of [j, −j, j . . . ]. This results in a clean, conceptual separation of I and Q samples between the real and imaginary paths for subsequent processing. The frequency response corruption of each ADC can then be associated with the real or imaginary data streams. A single Finite Impulse Response (FIR) filter is used to eliminate the corruption of the data paths, with the output of the filter being reassembled into one digital signal that includes all of the information of the original analog signal.
The conceptual separation of distortion into real and imaginary components provides the key to understanding that a single FIR filter can be implemented to correct for the frequency response mismatch of the two ADCs. However, the IF signal solution does not necessarily lead to a solution for correcting for frequency response mismatch between two ADCs in a system that digitizes a single analog baseband input signal. This is because a single analog baseband input signal cannot be separated into real and imaginary components.